Changing derailer changes balance

I brought my bike in for a tune up, (replace the tires)

He cut off but left the cable for the no longer made Shimano,
and added a fingertip resistance shifter). Maybe from having one
fingertip extention or the extra cable?

It seems much harder to control, and if I must push if from the
seat, it just won't work.

It was a short wheel Turner lite, which means one heavy bike!